The invention pertains to a method for transferring workpieces in which the workpiece is handled by a gripper-like holding device comprising at least one gripper arm mounted with freedom to pivot relative to a gripper carrier and in which the gripper arm is pivoted to perform gripping and release movements.
The invention also pertains to an apparatus for transferring workpieces comprising at least one gripper-like holding device for handling the workpiece, wherein the holding device is provided with at least one gripper arm mounted with freedom to pivot relative to a gripper carrier, the gripper arm comprising a first pivoted position for holding the workpiece and a second pivoted position for releasing the workpiece.
These types of methods and apparatuses can be used in conjunction with, for example, the production of containers by the blow-molding process. In these types of applications, the apparatus is installed in a blow-molding machine. The workpiece in question, which is handled by the holding device, can be a preform, a blow-molded container, or a support element which itself holds the preform or the blow-molded container.
In the molding of containers by the action of blown air, preforms of a thermoplastic material such as PET (polyethylene terephthalate) are sent to various processing stations within a blow-molding machine. A blow-molding machine of this type usually comprises a heating device and a blowing device, in which the previously tempered preform is expanded to form the container by a process of biaxial orientation. The expansion is produced by compressed air, which is introduced into the preform to be expanded. The course of the technical processes which take place during this type of preform expansion is explained in DE-OS 43 40 291. The previously mentioned introduction of the compressed gas also comprises the introduction of compressed gas into the developing container bubble as well as the introduction of compressed gas into the preform at the beginning of the blow-molding process.
The basic design of a blow-molding station for molding containers is described in DE-OS 42 12 583. Possible ways of tempering the preforms are explained in DE-OS 23 52 926.
Within the blow-molding machine, the preforms and the blown containers can be transported by means of various handling devices. The use of transport mandrels, onto which the preforms are set, is known. The preforms can also be handled by other types of carrying devices, however. The use of grippers to manipulate the preforms and the use of clamping mandrels, which can be inserted into the mouth of the preform to hold it, also number among the available designs.
A way of handling containers and preforms by the use of transfer wheels is described in, for example, DE-OS 199 06 438, according to which a transfer wheel is arranged between a blow-molding wheel and a discharge section, and another transfer wheel is arranged between the heating section and the blow-molding wheel.
The previously explained handling of the preforms takes place in a so-called two-stage process, in which the preforms are first produced by an injection-molding step and then stored temporarily. In the second stage, after a certain period of time, they are conditioned to the proper temperature and blow-molded into containers. A so-called single-stage process is also used, in which the preforms are produced by injection-molding, allowed to solidify sufficiently, suitably tempered, and then blow-molded immediately.
Various designs of the blow-molding stations are known. In the case of blow-molding stations which are arranged on rotating transport wheels, the mold carriers are often designed to open up like books. It is also possible, however, to use mold carriers which are guided in such a way that they can be shifted relative to each other or guided in some other way. In the case of stationary blow-molding stations, which are especially suitable for accepting multiple cavities for container molding, plates arranged parallel to each other are typically used as mold carriers.
The preforms can be handled either exclusively by grippers or by grippers in combination with other carrying or handling elements. For example, the preforms can be held by transport mandrels over a portion of their transport distance, after which they can be handled by grippers over at least one an additional portion of the transport distance. The grippers in use up to now, however, do not fulfill all of the requirements to be imposed on grippers to ensure the non-damaging and simultaneously reliable handling of the preforms.
The essential requirement is that the preforms must be handled reliably and without breakdowns at high transport rates per unit time. Advisably, the grippers should be able to handle both the preforms and the blow-molded bottles. In addition, mechanical damage to the preforms as a result of the handling by the grippers must be avoided or minimized.
Many different types of gripper-like holding devices are known. There are, for example, actively controlled grippers, in which the opening and closing movements are controlled by a mechanical cam control unit or by pneumatic or electrical actuating elements. There are also grippers in which the gripper arms are not actively controlled but rather in which the gripper arms are provided with slanted infeed surfaces, which, when the workpiece is pushed against them, cause the gripper arms to open. After the area of the workpiece to be gripped has been fully introduced, the grippers then snap back into place under the action of springs and thus hold the workpiece. In conjunction with the use of slanted guide surfaces, the workpiece can be removed again by the application of previously determined tensile forces.
Both the use of actively controlled grippers and the use of passively actuated grippers lead to advantages and disadvantages. Actively controlled grippers require suitable actuating elements, which take up space, lead to corresponding costs, and increase the amount of required maintenance work and servicing. Passively controlled grippers are usually very simple in design, but because of their direct contact with the workpieces, they lead to scratches or abrasion of the workpiece.